Pioneering documentary films made by the Post Office in the 1930s may look stilted and wooden now but they were actually the prototypes of a new type of film-making, a study co-authored by a Cambridge academic claims.

The short films, shown in cinemas before the main feature from the early 1930s onwards, were self-conscious advertising by the GPO. But their makers, led by the great documentary director John Grierson, also sought to show contemporary British life and believed that they should have a "socially useful purpose", inspired by Soviet film-makers such as Sergei Eisenstein.

The unit's best-known film is Night Mail, made in 1936, showing the overnight express carrying the post from London to Edinburgh. It is part-scripted by WH Auden with the famous refrain: "This is the night mail crossing the border/ Bringing the cheque and the postal order" with a score by Benjamin Britten. But it is only one of a series by the same group of film-makers working for several organisations, often raising social issues: Housing Problems, made for the British Commercial Gas Association in 1935 depicted the condition of Britain's slums, Enough to Eat from 1936 highlighted malnutrition.

Even potentially lighter films had a serious purpose: The Horsey Mail from 1938 featured postmen's heroic attempts to deliver mail through floods to the village of Horsey in East Anglia, taking to a rowing boat to ensure there was no delay.

Scott Anthony, a social and cultural historian at Christ's College, Cambridge, said: "People have tended to see the films quite narrowly. The film unit's output should be seen beyond that, in the context of its time. It is interesting in every direction: aesthetically, socially, culturally and politically.

"I don't think they had a particularly partisan political purpose, although many of the film-makers were on the left and Grierson himself was watched by the police. It was more a case of raising social issues, which the newsreels and newspapers were not doing, so that they could be discussed."

The film unit was set up in 1933 by Sir Stephen Tallents, the Post Office's head of public relations, to show the GPO and its 250,000 employees' role in society. But he also wanted to demonstrate how film could give people an honest picture of British society and bridge the gap between them and the institutions that affected their lives. The film-makers also chronicled the working lives of miners and fishermen in a distinctive style.

The message was one that might have gratified David Cameron – that members of society were interdependent and relied on pulling together. It was a conceit that paid dividends a few years later during the blitz.

The GPO's films – added this year to Unesco's memory of the world register and available as DVDs from the British Film Institute – show a picture of a society two generations ago that was almost entirely different from our own.

Many of the documentary makers joined the Crown Film Unit during the war and went on to work in advertising, films and television or for the Central Office of Information, which in the 1950s and 1960s produced the sort of instructional films teased by Harry Enfield's Mr Cholmondley-Warner.

"Just because those films became cliches does not mean that the original GPO documentaries were not interesting, dynamic and progressive," said Anthony.